Experiments carried out to date have continued to demonstrate the suitability of the guinea pig as an excellent animal model for the study of cytomegalovirus infection. Results obtained from the study of experimental guinea pig cytomegalovirus (GPCMV) infection have revealed a viremic stage during acute primary infection which continues for 4-10 days after inoculation. During the chronic persistent stage of the disease, demonstration of viremia in the presence of high levels of circulation antibody was accomplished only through in vivo assay which proved to be more sensitive than conventional in vitro methods. Transplacental transmission of GPCMV to the fetus was demonstrated, during acute primary infection but not in the chronic persistent stage of infection. Transmission of virus by common environmental contact was not as efficient as sexual contact between adult animals. Africian Green monkeys were commonly found to possess high levels of neutralizing antibody to the CSG strain of CMV, thus rendering them resistant to experimental infection with this virus. However, rhesus monkeys without pre-existing antibody could be infected experimentally; viruria persisted for months in the presence of circulating antibody. The immunofluorescent antibody test proved to be more sensitive than neutralization test for measuring antibody titers in monkeys.